1. Field of the Invention
This disclosure relates generally to the field of laser resonators and harmonic frequency generation, and in particular to intra-cavity and inter-cavity resonators for high efficiency harmonic generation.
2. Description of Related Art
Lasers have found broad application in many fields, such as medical procedures, scientific experiments, and industrial applications including marking objects, drilling, etc. The industrial demand for laser-machining of various materials in micrometer size has stimulated the research and development of ultraviolet (UV) beam generation, especially compact laser systems for generating high power UV laser beams.
Many of such UV lasers generate the UV output using an extra-cavity configuration; that is, a laser beam is generated in a cavity and the output of the cavity is directed to a crystal external to the cavity, such that the external crystal generates the UV output. Some lasers, however, generate a UV output using an intra-cavity configuration; that is, the laser beam and harmonics thereof are generated using mirrors and non-linear crystals internal to a cavity, which generates the UV output.
Some compact and commercially-oriented intra-cavity solid-state lasers have attained relatively high average UV output powers; for example, as discussed in A. J. Alfrey, "Intracavity Tripling of Diode-Pumped Nd:YVO4 at High Q-Switch Repetition Rates", CONFERENCE ON LASERS AND ELECTRO-OPTICS, pp. CPD19-1to CPD19-5 (1996). The Alfrey publication discloses a laser system which generates 355 nm laser radiation with an average UV output power of 2 W at 30 kHz. In addition, a solid-state laser system available from Lambda Physik is reported to be able to generate 4 W of average UV output power at 1 kHz.
For industrial applications, a need exists for compact solid-state laser systems providing even greater UV output power with a high enough repetition rate during operation to achieve high throughput in industrial processing.
Electromagnetic sum-frequency generation and difference-frequency generation in lasers has been known for over three decades for generating higher order harmonics. Hereinafter, NT.sup.TH order harmonic generation (HG) may be labelled NHG, in which N.gtoreq.1. For example, fundamental frequency generation may be labeled 1HG, second harmonic generation may be labelled 2HG, third harmonic generation may be labelled 3HG, etc.
Improvements in the power and efficiency of lasers have been achieved using non-linear media, such as lithium triborate (LBO) crystals, which perform sum-frequency processes. The non-linear nature of such non-linear media cause the conversion efficiency to increase as the intensity of the input frequencies increases.
One technique known in the art to increase the conversion efficiency is to implement the laser resonator in an intra-cavity configuration; for example, to place a non-linear crystal inside the laser resonator such that the laser intensity received by the crystal inside the laser is about one or two orders of magnitude higher than the output intensity of the laser. Such intra-cavity techniques may be used for second harmonic generation (labelled SHG or 2HG), as is known in the art, in which the second harmonics are obtained by generating the sum of fundamental frequencies.
Another sum-frequency generation technique is third harmonic generation (labelled THG or 3HG), which is one of the most efficient methods for generating UV wavelengths from solid-state lasers with a low M.sup.2 for the laser beam quality. Due to the non-linear nature of the elements employed, the conversion efficiency of THG techniques is generally proportional to the intensity of each of the two input frequencies; that is, the fundamental and the second harmonics. Traditional THG techniques in use have been implemented generally external to the laser cavity; that is, have been extra-cavity configurations, and have not utilized the existing higher intra-cavity intensities.
A typical and common problem of high intensity laser applications is damage to the crystal, such as a non-linear crystal, when focussing sharply on the crystal in order to achieve a desired conversion efficiency. Accordingly, a need exists for higher order harmonic generation which avoids such damage to the crystal and other laser components.
Some prior art lasers utilize the intra-cavity frequencies. Diode-pumped UV laser configurations using prisms are typical examples of intra-cavity SHG systems implementing THG in a one-pass configuration. For example, the A. J. Alfrey publication, described above, discloses such a system for generating 355 nm laser radiation with over 2 W average power, which creates subsequent THG in a two-pass configuration. However, the creation of THG from SHG in one-pass configurations in the prior art have generally resulted in relatively low efficiency.
Other intra-cavity techniques may be used, such as techniques described in U.S. Pat. No. 5,278,852 to Wu et al., in which SHG is performed in a sub-cavity, and THG is performed in a single-pass manner to operate as a low peak power laser. Such single-pass THG generally has less conversion efficiency than THG using a double-pass configuration. In addition, in implementing THG, intra-cavity techniques in the prior art may result in propagation of the UV beams back toward the lasing crystal, which may cause damage to other optical components in the main cavity. U.S. Pat. No. 5,025,446 to Kuizenga describes a high power laser system having an intra-cavity configuration, as well as back-propagation of the UV beams.
Accordingly, a need exists for higher order harmonic generation with improved conversion efficiency while avoiding back-propagation damage to the crystal and other laser components.
In addition, many lasers in the prior art are arranged in angular configurations, using sections configured, for example, at right angles to each other, and using mirrors and prisms for angularly directing the laser beams. The components positioned in such configurations are generally difficult to align, while linear configurations are relatively easier to align.
Accordingly, a need exists for a high power UV laser system having a linear configuration.